This patent application claims the benefit of the earlier filed. Chinese Patent Application serial number 200910262561.8 filed on Dec. 25, 2009.
This invention relates to a multilayer coating system derived from Titanium dioxide (TiO2) free aqueous coating compositions. The multilayer coating system comprises at least two layers including a layer of base coating which provides good hiding function for substrates, and a second layer of clear topcoat which provides surface protection functions for the coating system such as stain resistance, dirt pick up resistance, wet scrub resistance and solvent resistance with a soft touch feel and low gloss appearance.
TiO2 is a prime pigment in the architectural coating industry to provide hiding function for the substrates. Due to constant rise of TiO2 price and extremely energy consuming problem in TiO2 manufacture process, paint technologists are trying to reduce the use of TiO2 from long time ago. In white latex paints, TiO2 could be reduced in use by finding alternatives for TiO2 or reformulating paints above critical pigment volume concentration (CPVC) without sacrifice of their essential coating surface properties.
One typical example of above researches is to use calcined clay to partially replace TiO2 in latex coating formulations (Cost Effectiveness of Calcined Clays in Latex Paints, V. M. Braund, Polymers Paint and Colour Journal, Oct. 31, 1979). And later many other extender suppliers devoted to develop TiO2 replacements, such as ULTRACARB™ U5 (ULTRACARB™ U5 in Contract Matt Emulsion Paint, Micofine Minerals Limited, Paint & Resin, December, 1989). However, all these coating formulations disclosed still need an effective concentration of TiO2 to get acceptable opacity or hiding property while maintaining other surface properties such as stain resistance, wet scrub resistance, and etc.
Synthetic opaque polymer beads are useful opacifying agents, which can be selected from a variety of beads according to mechanical and/or optical requirements of the coating film and properties of the beads. Among these, beads containing microvoids introduce a synergistic effect which makes a significant contribution to the total light scattering for hiding function. Some opaque polymers were developed by Rohm and Haas Company as the optimum in polymeric extenders with a positive contribution to opacity without sacrifice of film properties, as disclosed in U.S. Pat. No. 4,427,836, U.S. Pat. No. 4,594,363, U.S. Pat. No. 4,880,842 and U.S. Pat. No. 4,970,241. EP 1,754,729 and EP 1,754,730 disclosed aqueous dispersions of film-forming polymeric particles which can provide a high degree of white light scattering when the film dries. However, above extenders and opaque polymers with or without binding capability were used as partial replacements for TiO2 in the literature and the coating formulations still need considerable TiO2 to contribute to hiding function while maintaining mechanical properties of the film. Otherwise, some important surface performance, such as hiding, gloss, burnish resistance and scrub resistance will be unacceptable.
There are challenges for the other way, reformulating paints above CPVC, which prone to generate more air voids between discrete and unconnected particles in the film. Such paints forms coating films containing interconnected air voids which cause increase of film porosity and decrease of paint qualities including wet scrub resistance, stain resistance and chalking resistance.
The problem addressed by this invention is to find a TiO2 free coating system possesses both a decent hiding property and a combination of surface properties including low gloss, soft touch, burnish resistance, easy clean function, dirt pick up resistance and solvent resistance.